Proof of one being a ninja, or proof of ninjaness.
This can be done by a photo of you as a ninja(face cant be fully shown, and camra man must die), giving a piece of knolage known only by ninjas, or disapearing like a ninja.
Failure to prove ones Ninjality will resualt in death or the title of a wannabee ninja... which is equal to death... only a ninja may ask for proof of Ninjality....